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Re: txv, piston

Originally posted by williesanchez69 I think the pston is more consistent,but this little piston can get stuck open or closed,the txv is better for eficientcie,better metering

Allow me to elaborate. A piston has one size opening, thats it. It's flow is strictly dependent on the liquid pressure and can provide flow on a properly charged system from as little as 5 degrees superheat to 30 degrees superheat depending on the load. The TXV has a needle valve that is operated by spring pressure, evaporator pressure and bulb pressure. It maintains a constant superheat level. A very simple machine that works well.

In both cases there is a moving part but frankly true failures with either are far and few between. More often than not, they are replaced or condemned on a guess.

FYI, I suspect you will see pistons go by the wayside when 13 SEER minimum comes around. Just curious but is anyone aware of a 13 SEER system with pistons?

Thanks for the helpful reply, doc. One question though. Doesn't a TXV also have an orifice at the end? So when it's fully open, there's a flow rate determined by that valve's orifice. Isn't that why there are different sized TXVs for different tonnage condensers? Won't a TXV for a 5 ton condenser will have a larger orifice that one for a 2 ton condenser?

Originally posted by wendel Thanks for the helpful reply, doc. One question though. Doesn't a TXV also have an orifice at the end? So when it's fully open, there's a flow rate determined by that valve's orifice. Isn't that why there are different sized TXVs for different tonnage condensers? Won't a TXV for a 5 ton condenser will have a larger orifice that one for a 2 ton condenser?

One might call the opening in which a TXV "needle" sits within an orifice. And yes, once the needle is fully removed from the orifice, the TXV would essentially act as a piston metering device. The difference is the TXV would only operate like that under full to extreme load conditions.

A piston runs like that all the time. In fact, as far as superheat is concerned, a piston metering device is like a stopped watch; it has the correct time only twice a day. A piston will give a system the correct superheat only under ideal conditions, which could actually occur as little as twice a day as outdoor temperatures rise and fall. During the night, the evap will tend to run a bit flooded, and during the high heat of the day, it'll run a bit starved. Neither is ideal for efficiency and longivity of the compressor.

Thanks for the explanation, Shophound. I think you corroborated the HVAC company's position. If the evaporator is oversized, the superheat will, most of the time, be greater than 10 degrees and will be full open giving a full flow of R-22. Therefore the TXV would be open and the flow limiting device would be the TXV's orifice.

TO clear up things the TXV has to have Out vap superheat of 12-15 deg to ensure there is no liquid refigerant after the TXV(this will cause the hunting not good) so dependant on the line set length and pressure drop you can be running a 25 deg sh @ comp. a Pisotn you can slightly over saturate the evaporator to lower the sh @ compressor.However the comment the piston is optimed in a couple of times a day is correct to some extent becouse at the ablient change so will the sh setting but it still can be just a efficient as a piston. But to answer the question YES bot need a liquid seal to opeteat correctly piston to keep it seated forward and the TWV to keep pressure on the spring so the o/e does not change.

The HP doesnt increase, the subcooling does. And a proper txv should not close under most conditions unless you are lacking airflow as mentioned. It merely throttles the refrigerant flow. They dont sit there and open and close, thats called hunting and usually means an oversized valve for the system.